In radio transmitters for broadcast, cellular, and satellite systems, the power amplifier (PA) in the transmitter has to be very linear, in addition to being able to simultaneously amplify many radio channels (frequencies) or independent user data channels, spread across a fairly wide bandwidth. It also has to do this efficiently, to reduce power consumption, need for cooling, and to increase its longevity. High linearity is required since nonlinear amplifiers would cause leakage of interfering signal energy between channels and distortion within each channel.
The amplitude probability density of a mix of sufficiently many independent radio frequency (RF) channels, or of a multi-user CDMA (Code Division Multiple Access) signal, tends to be close to a Rayleigh distribution having a large peak-to-average power ratio. Since a conventional linear RF power amplifier generally has an efficiency proportional to its output amplitude, its average efficiency is very low for such signals.
In response to the low efficiency of conventional linear power amplifiers when transmitting signals with large peak-to-average power ratio, two methods have been widely utilized: The Doherty method [1], and the Chireix outphasing method [2].
The Doherty amplifier uses one nonlinear and one linear amplifier. A first power amplifier is driven as a linear amplifier in class B, and a second power amplifier having nonlinear output current “modulates” the impedance seen by the first amplifier, through an impedance-inverting quarter-wave line [1, 3]. Since the nonlinear output current of the second amplifier is zero below a certain transition (output) voltage, the second amplifier does not contribute to the power loss below this voltage.
The standard Doherty amplifier's transition point (which corresponds to a maximum in the efficiency curve) is at half the maximum output voltage. The location of the transition point can be changed by changing the impedance of the quarter-wave transmission line (or equivalent circuit). Different size (power capacity) amplifiers will then be needed for optimum utilization of the available peak power. The Doherty system can be extended to three or more amplifiers, to obtain more maximum points on the efficiency curve. This usually leads to a requirement for very unevenly sized amplifiers (i.e. transistors).
The term “outphasing”, which is the key method in Chireix and LINC amplifiers, generally means the method of obtaining amplitude modulation by combining several (generally two) phase-modulated constant-amplitude signals. These signals are produced in a “signal component separator” (SCS) and subsequently, after up-conversion and amplification through RF chains (mixers filters and amplifiers), combined to form an amplified linear signal in an output combiner network. The phases of these constant-amplitude signals are chosen so that the result from their vector-summation yields the desired amplitude. The compensating reactances +jX and −jX in the output network of the Chireix amplifier are used to extend the region of high efficiency to include lower output power levels. The efficiency of Chireix systems are derived in [4, 5].
An advantage of the Chireix amplifier is the ability to change the efficiency curve to suit different peak-to-average power ratios, by changing the size (X) of the reactances. The peak output power is equally divided between the amplifiers irrespective of this adjustment, which means that equal size amplifiers can be used.
Furthermore, a three-transistor amplifier (or more generally an odd number of transistors) using the outphasing principle is described in [6]. However, adding more amplifiers to the Chireix amplifier, as suggested in [6], has so far been unsuccessful in increasing the efficiency. In fact, the amplifier described in [6] is less efficient than a conventional Chireix amplifier with modified drive signals, as described in [7].